Biomineralization in bryozoans: present, past and future

Abstract

Many animal phyla have the physiological ability to produce biomineralized skeletons with functional roles that have been shaped by natural selection for more than 500 million years. Among these are bryozoans, a moderately diverse phylum of aquatic invertebrates with a rich fossil record and importance today as bioconstructors in some shallow-water marine habitats. Biomineralizational patterns and, especially, processes are poorly understood in bryozoans but are conventionally believed to be similar to those of the related lophotrochozoan phyla Brachiopoda and Mollusca. However, bryozoan skeletons are more intricate than those of these two phyla. Calcareous skeletons have been acquired independently in two bryozoan clades - Stenolaemata in the Ordovician and Cheilostomata in the Jurassic - providing an evolutionary replicate. This review aims to highlight the importance of biomineralization in bryozoans and focuses on their skeletal ultrastructures, mineralogy and chemistry, the roles of organic components, the evolutionary history of bimineralization in bryozoans with respect to changes in seawater chemistry, and the impact of contemporary global changes, especially ocean acidification, on bryozoan skeletons. Bryozoan skeletons are constructed from three different wall types (exterior, interior and compound) differing in the presence/absence and location of organic cuticular layers. Skeletal ultrastructures can be classified into wall-parallel (i.e. laminated) and wall-perpendicular (i.e. prismatic) fabrics, the latter apparently found in only one of the two biomineralizing clades (Cheilostomata), which is also the only clade to biomineralize aragonite. A plethora of ultrastructural fabrics can be recognized and most occur in combination with other fabrics to constitute a fabric suite. The proportion of aragonitic and bimineralic bryozoans, as well as the Mg content of bryozoan skeletons, show a latitudinal increase into the warmer waters of the tropics. Responses of bryozoan mineralogy and skeletal thickness to oscillations between calcite and aragonite seas through geological time are equivocal. Field and laboratory studies of living bryozoans have shown that predicted future changes in pH (ocean acidification) combined with global warming are likely to have detrimental effects on calcification, growth rate and production of polymorphic zooids for defence and reproduction, although some species exhibit reasonable levels of resilience. Some key questions about bryozoan biomineralization that need to be addressed are identified.

Physical and Biological Determinants of the Fabrication of Molluscan Shell Microstructures

“…Lophophorates, bryozoans, and brachiopods secrete a large suite of calcitic microstructures (Williams, 1990(Williams, , 1997Taylor and Weedon, 2000;Taylor et al, 2015). Some cases display low degrees of organization, such as the granular microstructure of bryozoans or the fibrous primary layer of calcareous brachiopods, and they appear to be under little, if any, biological control.…”

Compositional Differences in the Habitat-Forming Bryozoan Communities of the Antarctic Shelf

“…However, no significant correlations have been found among pH, Mg-calcite content, and depth for species of Antarctic (n = 4, Figuerola et al, 2015) and Artic (n = 52, Borszcz et al, 2013) bryozoans, perhaps because polar bryozoans already exhibit lower levels of Mg-calcite content at both shallow and deeper depths. Despite these results, latitudinal patterns in skeletal composition suggest that there may be a more significant effect from temperature than from depth on aspects of the biomineralization process in bryozoans Taylor et al, 2009Taylor et al, , 2015. This assertion was not supported by more rigorous sampling of four species of Antarctic bryozoans that exhibited significant variability in the Mg-calcite skeletal content among sites and showed no significant correlation with temperature (Loxton et al, 2014).…”

“…Our study focuses on measuring the coverage and composition of the most abundant systematic groups of bryozoans occurring on the Antarctic shelf: the anascan Flustrina (flustrids) and two ascophoran groups, the Lepraliomorpha and Umbonulomorpha. We lumped the latter two systematic groups together, as recent phylogenetic analyses have determined the Lepraliomorpha and Umbonulomorpha to be paraphyletic clades that interdigitate (Waeschenbach et al, 2012;Taylor et al, 2015). Taking advantage of more recent imaging surveys with wider geographic distributions and betterresolved images of the Antarctic benthos, we used novel machine learning-based segmentation algorithms to discern between major morphological grades and clades of bryozoans.…”

In search of phylogenetic congruence between molecular and morphological data in bryozoans with extreme adult skeletal heteromorphy

“…There are two basic types of skeletal walls in cyclostomes (and other bryozoans): interior and exterior (see Taylor et al, 2014). Interior skeletal walls partition body cavities and are secreted by an epithelium that is present on both sides and wraps around the distal end of the wall.…”